A prior art scroll compressor 10 is shown in FIG. 5, and comprises a housing 12, a drive shaft 14 having a concentric shaft portion 16 and an eccentric shaft portion 18. The shaft 14 is supported at its concentric portion by bearings 20, which are fixed relative to housing 12, and driven by a motor 22. Second bearings 24 support an orbiting scroll 26 on the eccentric shaft portion 18 so that during use rotation of the shaft imparts an orbiting motion to the orbiting scroll 26 relative to a fixed scroll 28 for pumping gaseous fluid along a fluid flow path 30 between an inlet 31 and outlet 33 of the compressor.
Each scroll comprises a scroll wall 32, 34 which extends perpendicularly to a generally circular base plate 27, 29. The orbiting scroll wall 32 co-operates with the fixed scroll wall 34 during orbiting movement of the orbiting scroll. Scroll pumps are dry pumps and therefore the clearances between the scrolls must be accurately set during manufacture or adjustment to minimize seepage of fluid through the clearances. The phrase “dry pump” is well known in the art and is generally understood to mean a pump which does not contain any sealing or lubricating fluids exposed directly to vacuum in the pumping chamber.
In more detail, the space between the axial ends of a scroll wall of one scroll and the base plate of the other scroll is sealed by tip seals 36, but in order to allow the tip seals to seal effectively and to avoid excessive wear, the axial spacing between the orbiting scroll and the fixed scroll must be accurately controlled.
As the components of the compressor are manufactured within tolerances it is necessary when assembling the compressor to adjust the spacing between the orbiting scroll and fixed scroll to produce correct spacing in the axial direction. This procedure is commonly referred to as ‘shimming’.
In FIG. 5, the orbiting scroll is spaced from the fixed scroll with a spacer 38 positioned between a stepped portion of the drive shaft 14 and bearings 24. The spacer is generally circular and extends around a circumference of the eccentric portion 18 of the drive shaft. The axial thickness of the spacer 38 is selected to produce correct positioning of the orbiting scroll in an axial direction. When a spacer is located as shown the position of the orbiting scroll is shifted to the left in FIG. 5.
The FIG. 5 arrangement suffers from a numbers of problems. First, in order to determine if shimming is required it is normally necessary to inspect the compressor when it is partially or fully assembled. If adjustment of the spacing between the scrolls is required, it is necessary first to remove the fixed scroll, and then to remove the orbiting scroll. Subsequently, the bearing 24 must be removed and then finally a selected spacer can be located as shown in FIG. 5. Following this procedure the parts must be re-assembled and the compressor tested. If shimming is not correct, the procedure must be repeated. It will be appreciated that this process is unduly time consuming. It should also be noted that the procedure of removing and replacing parts of the compressor, such as the bearings 24 and the orbiting scroll 26, can in itself introduce small misalignments in the axial spacing of the orbiting scroll and the fixed scroll.
Secondly, the spacer itself must be accurately manufactured if it is not to produce misalignments within the compressor when it is inserted between the drive shaft 14 and the bearings 24. That is, if the end faces of the spacer are not parallel to each other, when the spacer is located in position it causes angular displacement of the orbiting scroll. Such angular displacement is referred to as swash. Swash causes an angle to be created between the scroll walls and consequently reduces efficiency as fluid is allowed to seep between the walls. Swash may also cause irregular spacing between the tip seals and the opposing scroll. The problems resulting from swash are further exacerbated because the spacer 38 is located relatively close in the radial direction to the central axis C of the compressor. Accordingly, if the end faces of the spacer are not parallel it produces a relatively large angular misalignment of the orbiting scroll.
The present invention seeks at least to mitigate one or more of the problems associated with the prior art.